ihe Marine Algca of the Firth of Forth. 427 



effect of any such factor of this kind as is ever introduced 

 of hut trifling moment. Any sudden variability either in 

 density of the water after freshets, or in its purity after 

 storms, seems also to be inadequate to account for such 

 phenomena. 



From a comparative point of view it becomes a matter 

 of considerable interest to compare the sizes, habit, and 

 appearance of th'e same species when growing at appa- 

 rently corresponding levels, but on substrata widely different 

 from one another. Thus many species infest submerged 

 wooden piles as well as floating blocks of wood (e.g., 

 Zflothrix flacca, Urospora penicilliformis, Enfcromorplca 

 compressa, Ectocarpi^ &c.) ; others are equally abundant 

 on submerged iron or steel (e.g., CalUthamnion corym- 

 hos2im, 0. graciUimum, Edocarpus siliculosus, Ceramium 

 rvJjrum, C. diaphamim, Foli/siphonice, &c.) ; while yet 

 other forms, which are almost invariably found growing in 

 other Algse or Epiphytes, are also sometimes found inde- 

 pendent on rocky ground, or vice versd {Polysiphonia 

 fastigiata, P. nigrescens, Mesogloia vermicular is, Griffithsia 

 corallina, &c.) In such circumstances averages of many 

 observations have shown that the finest results of growth 

 are found almost invariably in the positions normal to the 

 species in question — that is, in such places as are most 

 frequently occupied by the plants. The finest growths of 

 Polysiphonia fastigiata have invariably been found on 

 Fucus nodosns as host, while, on the other hand, Urospora 

 penicilliformis grows best on floating blocks of imported 

 wood. Ifesogloia vermicularis, which is most commonly 

 found on a rocky substratum, also occurs sometimes as an 

 Epiphyte on Polyides lumhricalis, but its finest development 

 has only been noted when found in the former position. In 

 the same manner Monostroma latissima is usually of small 

 size when found as an Epiphyte, though its host plants 

 are exceedingly abundant, and the same is the case 

 with Callithamiiion arhuscula, its hosts being exceedingly 

 few, while similarly by far the finest growths of CalUth- 

 amnion plumula have been found when the plant grew 

 in rocks — species obtained, especially on Ceramium 

 ruhrum, being all of but tiny size, and in a sterile con- 

 dition w^hen observed. A comparison of the results of 



